A significant amount of effort has been undertaken to use existing telephone lines for high speed data communications. As part of this effort, a number of Digital Subscriber Line (DSL) systems have been proposed. For example, a version known as Asymmetric Digital Subscriber Line (ADSL) provides a system that applies signals over a single twisted-wire pair that supports "plain old telephone service" (POTS) and high-speed duplex (simultaneous two-way) and simplex (from a network to the customer installation) digital services. Part of the proposed standard for ADSL is set forth in the Draft Proposed Revision of ANSI T1.413-1995--Interface Between Networks and Customer Installation--Asymmetric Digital Subscriber Line (ADSL) Metallic Interface (Sep. 26, 1997), which is hereby incorporated by reference.
A DSL system essentially encodes digital data as analog signals at very high data rates using special modems. Each user's link is conducted over twisted-pair conductors bundled with a large number of other twisted-pair conductors, each used at different times and for different purposes (e.g., voice only, data only, and both voice and data). The length of wire run from a user's remote transceiver to a central office transceiver may vary greatly from user to user. The physical environment of the system equipment varies over time due to temperature and humidity changes. A link may go online or offline at any time. Consequently, the analog DSL signals exists in a noisy, time varying environment. Accordingly, DSL systems use sophisticated equalizer training, echo canceling, and synchronization techniques to cope with these factors.
FIG. 1 is a block diagram of one embodiment of a prior art DSL system. A user's computer 10 is coupled to a DSL modem 12 through a bandwidth splitter 14 to a conventional telephone line 16 and thence to a telephone company (telco) system 18. The telco system 18 includes a DSL modem and necessary equipment to establish a link to, for example, the Internet. The splitter 14 separates voice band frequencies from higher data band frequencies. A conventional telephone 20 may be coupled to the splitter 14 for communication over the voice band frequencies.
The DSL modem 12 may include an internal processor to control overall operation of the modem, or may be controlled by the user's computer 10 running appropriate software (a "soft modem"). In the telco system 18, each DSL modem is typically implemented with a programmable digital signal processor (DSP) 22.
One characteristic of most DSL systems, and ADSL in particular, is that the probability is high that each user link will operate in an "always on" or "always connected" mode, and will be used primarily to transport Internet traffic. However, it is unlikely that any particular link will be in essentially constant use transmitting data. Thus, it is likely that a link will remain idle for extended periods of time during user inactivity and will transport blocks of data generated in bursts during user activity.
During idle time, a number of problems occur if no data is transmitted over a connected link. Synchronization between a user's remote transceiver and a central office transceiver may be lost since no signal is being sent. Training and echo canceling parameters may become "dated", since they are dynamically generated in response to varying conditions over the transport medium (conductor bundle). Hence, when a link exits idle mode, retraining would be required, reducing the responsiveness of the link.
One approach to overcoming this problem is to continuously send a sequence of meaningless data, such as all binary ones, from the processor controlling the modem 12. However, under present systems, encoding even a constant series of data signals results in a time-varying set of modulated symbols being output from the modem 12. Consequently, the receiving modem sees varying data that must be decoded and analyzed before a decision can be made that the data represents an idle code or signal. This requires significant computational power. Thus, for the telco system 18, each link to a user's computer 10 generally requires a dedicated modem. One consequence of this is that implementing a DSL system is expensive in terms of capital equipment. Another consequence is that electrical power consumption is high, since power usage in a DSP varies with computational requirements. In some telco installations, the amount of available electrical power is limited, thus limiting the number of modems that can be installed.
Accordingly, the inventors have determined that it would be useful to have a system and method for indicating an idle state in a DSL communication system that does not result in a loss of synchronization or require retraining, and which reduces processing load and electrical power consumption. The present invention provides such a system and method.